// funcao-01.js
const cor_default = "#444";
const cor_um = "orange";
const cor_dois = "moccasin";
const btnDefault = document.querySelectorAll(".cor")[0];
const btnUm = document.querySelectorAll(".cor")[1];
const btnDois = document.querySelectorAll(".cor")[2];
// ----------------
btnUm.insertAdjacentHTML("afterbegin", `${cor_um}`);
btnDois.insertAdjacentHTML("afterbegin", `${cor_dois}`);
// ----------------
btnDefault.addEventListener("click", corDefault);
btnUm.addEventListener("click", corUm);
btnDois.addEventListener("click", corDois);
// ----------------
// Funções
function corDefault() {
  document.body.style.backgroundColor = cor_default;
}
function corUm() {
  document.body.style.backgroundColor = cor_um;
}
function corDois() {
  document.body.style.backgroundColor = cor_dois;

<button type="button" class="cor">default</button>
<button type="button" class="cor"></button>
<button type="button" class="cor"></button>
<script src="js/funcao-01.js"></script>

Função mudaCor()

default